Examples of electrical components having a ceramic base body and internal electrodes are thermistors and varistors. The base body of varistors is frequently produced from a mixture of different metal oxides, for example, from zinc oxide. Varistors demonstrate a non-linear, voltage-dependent change in resistance, which is used to protect an electronic circuit against over-voltage. In this process, the resistance value of varistors decreases with an increasing voltage that is applied.
In the diversion of electrical over-voltage pulses by the varistor, the energy of the pulse must be absorbed by the ceramic base body. Particularly in the case of short voltage pulses, strong local heating of the ceramic base body occurs between the internal electrodes. In this process, temperature equalization with the environment is not possible. This results in great temperature differences, which bring about mechanical stresses in the ceramic base body. In this process, the internal electrode causes further weakening of the cross-sectional area of the ceramic base body. For this reason, cracks can occur in the ceramic body, in case of severe local heating of the varistor, and these can result in ceramic parts coming loose from the base body.
Conventional varistors have two contact surfaces on the faces of their base bodies. Internal electrodes are arranged in the interior of the base body, which, in cross-section, are pushed into one another in chamber-like manner through all levels, wherein the internal electrodes are each alternately connected with a contact surface of different polarity (see FIG. 1A). A cross-section through the base body of the varistor along a ceramic layer shows that a major portion of the surface of this layer is taken up by the internal electrode (see FIG. 1b). This results in weakening of the ceramic body in this region, so that in case of a large electrical pulse, crack formation must be expected in the ceramic base body, due to over-heating.
A different variant of varistors is known from the publication DE 199 31 056 A1, in which internal electrodes are arranged in such a manner that two electrode stacks that do not overlap with one another are formed, in each instance, each of which contacts a contact surface (see FIG. 2A). A cross-section through the ceramic base body along a ceramic layer shows that two internal electrodes 15A, 15B, which lie opposite one another, are arranged on a ceramic layer (see FIG. 2B). In the case of a large electrical pulse, local over-heating occurs also in the case of varistors having this construction, and this can result in fracture of the base body.
An NTC thermistor element is known from the published patent application DE 198 06 296 A1, which has at least one ceramic layer on which a longer and a shorter internal electrode are arranged, the ends of which are opposite one another, spaced apart. In this arrangement, the internal electrodes can overlap, if they are connected with the same contact surface arranged on the outer surface of the component, or also internal electrodes can overlap if they are connected with different contact surfaces. In the case of this thermistor element, as well, crack formation in the ceramic base body can be expected in case of a large electrical pulse.